For large buildings, it is common to install an air conditioning device to maintain the room under appropriate temperature and humidity conditions. The air conditioning device may cool or warm the air flow created by the supply fan of the air handling unit. The cooled or warmed air may move along the duct and be supplied to the room that needs air-conditioning. In this case, since a loud noise is generated by the supply fan, if the noise is not properly controlled, it may be transmitted to the room and thus, the air conditioning device cannot be practically used. Therefore, it is required to install a sound attenuator for reducing noises in the middle of the duct.
The sound attenuator is a device to reduce or eliminate loud noises. The sound attenuator may be classified into an intake sound attenuator and an exhaust sound attenuator according to the installation location. A muffler which is often used in automobiles or motorcycles is a kind of the exhaust sound attenuator. Moreover, the sound attenuator may be classified into a sound absorption type sound attenuator, an interference type sound attenuator, an expansion type sound attenuator, and a resonance type sound attenuator, and so on according to the sound attenuation method. The sound attenuator which is commonly used in the air conditioning device includes a sound absorber formed in the duct to absorb and reduce noises.
FIG. 1A is a schematic view for illustrating a sound attenuator for an air conditioning device in accordance with a comparative example, and FIG. 1B is a schematic view of a cross-section taken along the line A-A of FIG. 1A.
Referring to FIGS. 1A and 1B, a sound attenuator 10 in accordance with the comparative example may include a protective case 11, a splitter 12 and an air passage 13. The protective case 11 may be formed of a metal, etc. And, the splitter 12 for absorbing sound and the air passage 13 through which air can pass may be formed in the protective case 11. The air flow may be indicated by an arrow in FIG. 1B.
The splitter 12 may include a porous sound absorber 21 and an outer frame 22 surrounding the porous sound absorber 21.
The porous sound absorber 21 may have a plurality of pores or a fibrous structure on the surface of the absorber 21 and inside the absorber 21 to be vibrated by the sound waves of air. As a result of such vibration, friction between materials may occur and sound energy may be converted into heat energy and absorbed.
The outer frame 22 may be formed of a perforated plate which is a thin plate having a plurality of pores.
The splitter 12 may be formed in various shapes other than the shape shown in FIG. 1B.
Performances of the sound attenuator may be expressed as the sound reduction index per unit length. The sound reduction index is proportional to the sound absorbing ratio. As an example, when the attenuator is formed in a lined duct, the sound reduction index R (dB) may be calculated as follows:
      R    =                  K        ·                  P          S                    ⁢      l            wherein    ⁢          :                  K      =              4.34        ⁢                  1          N                ⁢                                  ⁢                  (                                    Provided              ⁢                                                          ⁢              that              ⁢                                                          ⁢              wa              ⁢                              /                            ⁢                              N                c                                      <            1                    )                      ,                  P: Cross-sectional length of lined duct (m),        S: Area of lined duct (m2),        l: Length of lined duct (m),        N: Ratio of maximum sound pressure and minimum sound pressure when measuring the normal incidence sound absorbing ratio α0 in the impedance tube,        w: Angular frequency (=2πf),        f: Frequency (Hz),        a: Side length of lined duct (m),        C: Sound velocity (m/s).        
Moreover, α0 and N have a relationship represented by the following formula:
      α          0      =        ⁢      1          N      +              N                  -          1                    +      2      
Accordingly, as the N value is increased, the sound absorbing ratio α0 value is decreased and the sound reduction index per unit length is also decreased. Therefore, it is very important to increase the sound absorbing ratio in the sound attenuator including the splitter because increasing the sound absorbing ratio directly affects the performance of the sound attenuator.
Meanwhile, the sound absorber may be classified into three types, that is, a porous sound absorber, a resonance sound absorber and a plate-type sound absorber according to the method and technical feature of sound absorption.
The porous sound absorber has a plurality of small bubble-like pores or a fibrous structure on the surface of the absorber and inside the absorber, so that it is vibrated by the sound waves of air. As a result of such vibration, friction between materials may occur and sound energy may be converted into heat energy and absorbed. The main sound absorption range of the porous sound absorber may be the high-frequency range of 250 Hz or more. The porous sound absorber may be formed of glass wool, rock wool, foamed resin materials, and fabrics, and so on.
The resonance sound absorber uses a principal of Helmholtz Resonators and is a container of gas (usually air) with an open hole (or neck or port). A volume of air in and near the open hole vibrates because of the ‘springiness’ of the air inside. When the sound wave with a resonance frequency arrives, it is possible to absorb acoustic energy due to viscous resistance of the open hole. The main sound absorption range of the resonance sound absorber may be the middle frequency range of 125 Hz to 250 Hz. The resonance sound absorber may include a perforated plate having an air layer formed on the rear side of the plate.
The plate-type sound absorber uses resonance due to vibration of the plate. The plate-type sound absorber may exhibit the sound absorbing effect by converting acoustic energy into vibrational energy when a sound wave vibrates the plate. The main sound absorption range of the sound absorber using resonance due to vibration of the plate may be the low frequency range of 125 Hz or less. The plate-type sound absorber may be formed of a metal plate, a vinyl film, and a gypsum wallboard, and so on.
The porous sound absorber is commonly used in the sound attenuator for the air conditioning device. The performance of the porous sound absorber depends on porosity or a thickness of the sound absorber. It is possible to increase the sound absorbing ratio if the thickness of the sound absorber is increased.
Since the noise generated in the air conditioning device is affected by the rotation of the fan, the noise with a frequency of 125 Hz or less is dominant. In order to reduce the noise with a frequency of 125 Hz or less, a thickness of the porous sound absorber in the sound attenuator should be increased. However, it is limited to increase the thickness of the sound absorber in a low-frequency range of 125 Hz or less.
If the porous sound absorber has the sound absorbing ratio of 99%, that is, the sound absorption coefficient of 0.99, it may be referred to as the perfect sound absorber. It is known that in order to achieve such a sound absorption coefficient, the thickness of the absorber should be increased to ¼ wavelength. It can be simply calculated as follows:
TABLE 1⅓ octave center frequency (Hz)40506380100125¼ wavelength (m)2.11.71.351.060.850.68
Accordingly, in order to reduce the noise in the frequency region of 125 Hz or less, it is required to increase the thickness or the length of the splitter. Since the cross-sectional length of the lined duct is usually 1 m or less and the area of the air passage in the sound attenuator for the lined duct is usually about 50% of the total area, the thickness of the splitter may be usually 150 to 300 mm. Therefore, it is significantly difficult to set the sound absorbing ratio at 125 Hz or less to be 0.6 or more by using only the porous sound absorber included in the splitter.
In this context, although it is not a method for increasing the sound absorbing ratio in the sound attenuator, a method for increasing the sound absorbing ratio by combining a porous sound absorber with a sound absorber using resonance due to vibration of a metal plate has been presented (Patent Literature 1).
However, in case of the sound absorber using resonance due to vibration of a plate such as the technique disclosed in Patent Literature 1, since a difference in the sound absorbing ratio may occur depending on a method for fixing edges of the plate or a method for coupling the plate to the sound absorber, it is very difficult to control properties such as the sound absorbing ratio. Therefore, it is not suitable for use in the sound attenuator. Moreover, there is a limitation in increasing the weight or size of the metal plate in the plate-type absorber disclosed in Patent Literature 1 so that there is also limitation in arbitrarily adjusting the resonance frequency.
Further, the plate-type absorber disclosed in Patent Literature 1 includes the metal plate and a rear polymer board. The metal plate should be bonded to the rear polymer board via adhesion and the like. However, since most porous sound absorbers used for the rear polymer board tend to fall off easily due to the weight of the rear polymer board itself when attached to the metal plate, the sound absorber used for the rear polymer board that can be bonded to the metal plate is very limited. For this reason, in Patent Literature 1, the rear polymer board that can be bonded to the metal plate is limited to melamine resin foam that can prevent falling due to the weight.
Moreover, in the plate-type sound absorber, the edges of the metal plate should be fixed consistently. If the porous sound absorbers located on or below the metal plate press the metal plate, the sound absorbing ratio may change.
Meanwhile, regarding the sound attenuator for air ducts, a resonance-type splitter which allows absorption of sound due to the resonance effect of the Helmholtz resonator has been presented (Patent Literature 2). The splitter disclosed in Patent Literature 2 includes an intermediate plate (sound absorbing plate) formed in the center of the splitter and a resonance plate spaced from the intermediate plate and formed on both sides of the intermediate plate. In the space between the intermediate plate and the resonance plate, the sound entering the hole of the resonance plate is extinguished due to the resonance effect, and the remaining sound which is not extinguished due to the resonance effect is absorbed by the intermediate plate to be extinguished.
However, in the technique disclosed in Patent Literature 2, there is only the resonance effect of the perforated plates formed on both sides of the intermediate plate, but there is no relation between the spaces on both sides of the intermediate plate. That is, since the space on one side of the intermediate plate does not play an additional role in the sound absorbing effect in the space on the other side of the intermediate plate, as a result, each of the perforated plates provided on both sides of the intermediate plate may only serve as a separate sound-absorbing layer. Accordingly, the sound absorbing effect of the perforated plate is limited to a very narrow range, i.e., the resonance frequency of Helmholtz resonance. In this case, in order to increase the sound absorbing ratio, it is required to increase the thickness or length of the sound absorber constituting the intermediate plate, and thus the thickness or length of the splitter. However, since the size of the sound attenuator for air ducts is limited as described above, it is practically difficult to increase the sound absorbing ratio by increasing the thickness or length of the splitter.
Therefore, it is still required to develop techniques for improving the sound absorbing property of the sound attenuator of the air conditioning device, in particular, in the low frequency region of 125 Hz or less.